1. Field of the Invention
The present invention relates to an electric motor.
2. Description of the Related Art
Conventional electric motors, which are provided with a first rotor and a second rotor in a concentrically circular arrangement around a rotational axis of the electric motor and which control the phase difference, that is, the relative positions in the circumferential direction of the first rotor and the second rotor in response to the rotational speed of the electric motor, or in response to the speed of rotating magnetic field generated in the stator, are well known (for instance, refer to Japanese Unexamined Patent Application, First Publication No. 2002-204541).
For instance, to control the phase difference of the first rotor and the second rotor in response to the rotational speed of the electric motor, the relative positions in the circumferential direction of the first rotor and the second rotor are changed in this electric motor through members that displace along the radial direction under the action of centrifugal force. For example, to control the phase difference of the first and the second rotors in response to the speed of the rotating magnetic field generated in the stator, a control current is passed through the stator coil with the rotational speed maintained by the inertia of each rotor, and the speed of the rotating magnetic field is changed so as to change the relative positions in the circumferential direction of the first and second rotors.
Incidentally, return springs are used to restore the relative positions of the first and second rotors to their home positions in the electric motor according to one of the instances of the prior art mentioned above. This results in the problem of increasing the number of parts and the cost thereof.
The present invention was made in view of above circumstances and has an object of providing an electric motor which can reduce the cost thereof by reducing the number of parts.